github.com/meulengracht/snapd@v0.0.0-20210719210640-8bde69bcc84e/cmd/snap-discard-ns/snap-discard-ns.c

/*
 * Copyright (C) 2015-2019 Canonical Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#define _GNU_SOURCE

#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <fnmatch.h>
#include <limits.h>
#include <linux/magic.h>
#include <stdio.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/vfs.h>
#include <unistd.h>

#include "../libsnap-confine-private/error.h"
#include "../libsnap-confine-private/locking.h"
#include "../libsnap-confine-private/snap.h"
#include "../libsnap-confine-private/string-utils.h"
#include "../libsnap-confine-private/utils.h"

#ifndef NSFS_MAGIC
#define NSFS_MAGIC 0x6e736673
#endif

int main(int argc, char** argv) {
    if (argc != 2 && argc != 3) {
        printf("Usage: snap-discard-ns [--from-snap-confine] <SNAP-INSTANCE-NAME>\n");
        return 0;
    }
    const char* snap_instance_name;
    bool from_snap_confine;

    if (argc == 3) {
        if (!sc_streq(argv[1], "--from-snap-confine")) {
            die("unexpected argument %s", argv[1]);
        }
        from_snap_confine = true;
        snap_instance_name = argv[2];
    } else {
        from_snap_confine = false;
        snap_instance_name = argv[1];
    }

    sc_error* err = NULL;
    sc_instance_name_validate(snap_instance_name, &err);
    sc_die_on_error(err);

    int snap_lock_fd = -1;
    if (from_snap_confine) {
        sc_verify_snap_lock(snap_instance_name);
    } else {
        /* Grab the lock holding the snap instance. This prevents races from
         * concurrently executing snap-confine. The lock is explicitly released
         * during normal operation but it is not preserved across the life-cycle of
         * the process anyway so no attempt is made to unlock it ahead of any call
         * to die() */
        snap_lock_fd = sc_lock_snap(snap_instance_name);
    }
    debug("discarding mount namespaces of snap %s", snap_instance_name);

    const char* ns_dir_path = "/run/snapd/ns";
    int ns_dir_fd = open(ns_dir_path, O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
    if (ns_dir_fd < 0) {
        /* The directory may legitimately not exist if no snap has started to
         * prepare it. This is not an error condition. */
        if (errno == ENOENT) {
            return 0;
        }
        die("cannot open path %s", ns_dir_path);
    }

    /* Move to the namespace directory. This is used so that we don't need to
     * traverse the path over and over in our upcoming umount2(2) calls. */
    if (fchdir(ns_dir_fd) < 0) {
        die("cannot move to directory %s", ns_dir_path);
    }

    /* Create shell patterns that describe the things we are interested in:
     *
     * Preserved mount namespaces to unmount and unlink:
     * - "$SNAP_INSTANCE_NAME.mnt"
     * - "$SNAP_INSTANCE_NAME.[0-9]+.mnt"
     *
     * Applied mount profiles to unlink:
     * - "snap.$SNAP_INSTANCE_NAME.fstab"
     * - "snap.$SNAP_INSTANCE_NAME.[0-9]+.user-fstab"
     *
     * Mount namespace information files:
     * - "snap.$SNAP_INSTANCE_NAME.info"
     *
     * Use PATH_MAX as the size of each buffer since those can store any file
     * name. */
    char sys_fstab_pattern[PATH_MAX];
    char usr_fstab_pattern[PATH_MAX];
    char sys_mnt_pattern[PATH_MAX];
    char usr_mnt_pattern[PATH_MAX];
    char sys_info_pattern[PATH_MAX];
    sc_must_snprintf(sys_fstab_pattern, sizeof sys_fstab_pattern, "snap\\.%s\\.fstab", snap_instance_name);
    sc_must_snprintf(usr_fstab_pattern, sizeof usr_fstab_pattern, "snap\\.%s\\.*\\.user-fstab", snap_instance_name);
    sc_must_snprintf(sys_mnt_pattern, sizeof sys_mnt_pattern, "%s\\.mnt", snap_instance_name);
    sc_must_snprintf(usr_mnt_pattern, sizeof usr_mnt_pattern, "%s\\.*\\.mnt", snap_instance_name);
    sc_must_snprintf(sys_info_pattern, sizeof sys_info_pattern, "snap\\.%s\\.info", snap_instance_name);

    DIR* ns_dir = fdopendir(ns_dir_fd);
    if (ns_dir == NULL) {
        die("cannot fdopendir");
    }
    /* ns_dir_fd is now owned by ns_dir and will not be closed. */

    while (true) {
        /* Reset errno ahead of any call to readdir to differentiate errors
         * from legitimate end of directory. */
        errno = 0;
        struct dirent* dent = readdir(ns_dir);
        if (dent == NULL) {
            if (errno != 0) {
                die("cannot read next directory entry");
            }
            /* We've seen the whole directory. */
            break;
        }

        /* We use dnet->d_name a lot so let's shorten it. */
        const char* dname = dent->d_name;

        /* Check the four patterns that we have against the name and set the
         * two should flags to decide further actions. Note that we always
         * unlink matching files so that is not reflected in the structure. */
        bool should_unmount = false;
        bool should_unlink = false;
        struct variant {
            const char* pattern;
            bool unmount;
        };
        struct variant variants[] = {
            {.pattern = sys_mnt_pattern, .unmount = true},
            {.pattern = usr_mnt_pattern, .unmount = true},
            {.pattern = sys_fstab_pattern},
            {.pattern = usr_fstab_pattern},
            {.pattern = sys_info_pattern},
        };
        for (size_t i = 0; i < sizeof variants / sizeof *variants; ++i) {
            struct variant* v = &variants[i];
            debug("checking if %s matches %s", dname, v->pattern);
            int match_result = fnmatch(v->pattern, dname, 0);
            if (match_result == FNM_NOMATCH) {
                continue;
            } else if (match_result == 0) {
                should_unmount |= v->unmount;
                should_unlink = true;
                debug("file %s matches pattern %s", dname, v->pattern);
                /* One match is enough. */
                break;
            } else if (match_result < 0) {
                die("cannot execute match against pattern %s", v->pattern);
            }
        }

        /* Stat the candidate directory entry to know what we are dealing with.
         */
        struct stat file_info;
        if (fstatat(ns_dir_fd, dname, &file_info, AT_SYMLINK_NOFOLLOW) < 0) {
            die("cannot inspect file %s", dname);
        }

        /* We are only interested in regular files. The .mnt files, even if
         * bind-mounted, appear as regular files and not as symbolic links due
         * to the peculiarities of the Linux kernel. */
        if (!S_ISREG(file_info.st_mode)) {
            continue;
        }

        if (should_unmount) {
            /* If we are asked to unmount the file double check that it is
             * really a preserved mount namespace since the error code from
             * umount2(2) is inconclusive. */
            int path_fd = openat(ns_dir_fd, dname, O_PATH | O_CLOEXEC | O_NOFOLLOW);
            if (path_fd < 0) {
                die("cannot open path %s", dname);
            }
            struct statfs fs_info;
            if (fstatfs(path_fd, &fs_info) < 0) {
                die("cannot inspect file-system at %s", dname);
            }
            close(path_fd);
            if (fs_info.f_type == NSFS_MAGIC || fs_info.f_type == PROC_SUPER_MAGIC) {
                debug("unmounting %s", dname);
                if (umount2(dname, MNT_DETACH | UMOUNT_NOFOLLOW) < 0) {
                    die("cannot unmount %s", dname);
                }
            }
        }

        if (should_unlink) {
            debug("unlinking %s", dname);
            if (unlinkat(ns_dir_fd, dname, 0) < 0) {
                die("cannot unlink %s", dname);
            }
        }
    }

    /* Close the directory and release the lock, we're done. */
    if (closedir(ns_dir) < 0) {
        die("cannot close directory");
    }
    if (snap_lock_fd != -1) {
        sc_unlock(snap_lock_fd);
    }
    return 0;
}